Triplen harmonics
BSRIA Power quality guide (AG 2/2000) was written by C C Pearson and V Uthayanan and published by BSRIA in July 2000. It states:
The currents in the phases of a three phase supply are 120° out of phase with each other, and their vector sum in a combined neutral is reduced to zero in a balanced system. The third harmonic has peaks at 120° phase angles. When these combine in the neutral, the peaks are coincident, so the resultant harmonic current is the sum of the phase harmonic currents. Multiples of the third harmonic also have peaks at 120° phase angles which will likewise be coincident in the neutral, giving rise to addition of the currents. These multiples of the third harmonic are known as triple-n or triplen harmonics. The even multiples of the third harmonic are rarely present, even harmonics are absent from symmetrical waveforms. Therefore the term triplen is often used to refer only to the odd multiples of the third harmonic.
--BSRIA
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